Goldman's 2015 forecast: 'Flat is the new up'

The S&P 500 is up just more than 1 percent in 2015, trickling along in some of the most listless activity many traders have ever witnessed.

Goldman Sachs told clients in a note Friday not to expect much more excitement the rest of the year.

With the market up 18 percent annualized in the last three years and 13 percent in the last five years, which is way above the long-term average, the market is due for some mean reversion, notes Goldman's David Kostin.

"The most likely path of the U.S. stock market during the next six months is sideways," wrote Kostin. "Put simply, 'flat is the new up.'"

Here are four reasons Goldman expects the market to stay flat and what it's telling clients they should do about it...